This invention relates to improvements in electronic circuitry used in moving read/write heads in a memory disc system for use with computers, and, more particularly, to improvements in such circuitry for providing drive signals to a voice coil motor for such a system.
Voice-coil motors are linear actuators that are widely used for moving heads and their support assemblies across discs in computer system disc drives in order to read data from or write data to the disc and in activating or deactivating disc drives. The heads float across the disc surface on a cushion of air resulting from rotation of the discs. In a conventional disc drive, the disc is roughened on at least portions of the disc surface to obviate sticking of the head to the disc surface as the disc is spun from a stop to an operating speed.
As data densities on the discs have increased, need for greater precision and accuracy in head positioning has also increased. Additionally, spacings between the heads and the discs have decreased to a point where roughening of the disc surface is impractical. As a result of these changes, a prior art practice of xe2x80x9cparkingxe2x80x9d the head in the innermost data track in an area removed from areas of the disc that store data no longer provides adequate safeguarding of the head or of the disc when the computer system is not in use and particularly when the head is deployed from the parked position.
In increasing numbers of disc drives, the head is parked by causing the head support assembly to traverse a ramp to remove the head from proximity to the disc when the disc drive is deactivated as the system is shut down. When the head support assembly reaches the end of the ramp, the head support assembly is latched into a storage position. The head then cannot collide with the disc if the disc drive is jarred or bumped, avoiding one potential source of damage to the head or to the disc.
As the system is reactivated, the head is unparked by releasing the head support assembly from the latch. The head support assembly then traverses the ramp towards the disc in response to signals delivered to the voice coil motor from a controller. The head must be moving with the correct speed when the head support assembly reaches the end of the ramp in order to maintain the head in proximity to the disc without collision between the head and the disc. As a result, the controller must provide the proper drive signals to the voice coil motor resulting in the correct speed for the head when the head support assembly exits the ramp.
One method for driving the voice coil motor is to apply a constant voltage to a voice coil in the voice coil motor. However, the voice coil motor generates a back electromotive force (BEMF) because the voice coil is moving in a magnetic field. The actual voltage driving the voice coil motor thus is the sum of the resistive voltage (I.R.) and the BEMF, which varies with voice coil motor velocity VM. As a result, the applied voltage is not the actual BEMF of the voice coil motor.
Some conventional voice coil motor controller circuits employ a digital to analog converter circuit for providing analog control signals to the voice coil motor controller in response to digitally preprogrammed profiles. However, these conventional controller circuits have limited ability to compensate for wearing of the ramp and of the portions of the head supporting assembly that are in contact with the ramp. Additionally, conventional controller circuits have limited capability for providing control signals responsive to head velocity variations originating from other sources, such as motion of the disc drive.
In prior art approaches to driving voice coil motors and compensating for the BEMF, as described in U.S. Pat. Nos. 5,566,369 and 5,297,024, both issued to F. Carobolante, a current sensing resistor is coupled in series with the voice coil motor. A differential buffer amplifier has inputs coupled to the terminals of the current sensing resistor and provides an output signal that is proportional to a current through the voice coil motor. A comparison circuit then allows the current through the voice coil motor to be corrected to a desired value. However, the effective resistance of the voice coil motor causes some of the energy from the current through the voice coil motor to be lost as heat. As a result, this form of feedback, while providing improved performance for the voice coil motor, does not result in optimal performance, especially as voice coil motor characteristics change with age, temperature and the like.
In several aspects, the present invention includes circuits and methods for providing feedback from the motion of a head to a voice coil motor controller circuit to correct head velocity during ramp loading of the head from a disc into a storage position and particularly during ramp unloading from the storage position into proximity to the disc. As a result, voice coil motor velocity may be monitored and corrected to compensate for temperature-induced mechanical changes and also for wear of moving components that are in contact with other components.
In one aspect, the present invention includes a power supply circuit coupled to the voice coil motor that in turn is coupled to the head. A controller provides signals to the voice coil motor to correct voice coil motor velocity in response to signals from a feedback network. The feedback network includes a sample and hold circuit that is coupled to the voice coil motor during intervals when the power supply circuit is not providing drive signals to the voice coil motor.
In another aspect, the present invention includes a method for driving a voice coil motor in response to signals from a feedback network that senses voice coil motor velocity. The method includes steps of providing a drive signal to an H-bridge for a first interval. At the end of the first interval, the H-bridge is placed in a high impedance state. Following a pause during a second interval while transient voltages extinguish, a sample and hold circuit is coupled to the voice coil motor. The sample and hold circuit measures a voltage from the voice coil motor that is directly proportional to voice coil motor velocity and thus is directly related to head velocity. After the sample and hold circuit measures the voice coil motor voltage, the input to the sample and hold circuit is disabled. An output signal from the sample and hold circuit is coupled to the feedback network and thus to the H-bridge. As a result, head velocity is more accurately controlled, reducing probability of collision between the heads and the discs and thereby increasing reliability of the disc drive.